1. Field of the Invention
The present invention generally relates to spacecraft and, in particular, relates to improved performance, high-efficiency spacecraft propulsion.
2. Description of the Related Art
For Global Positioning System (“GPS”) III missions, it is desirable to dual-launch spacecraft into a low Earth orbit (“LEO”) storage orbit and then transfer the spacecraft to a medium Earth orbit (“MEO”) at a later time. This approach would enable dual-launched spacecraft to be delivered to separate orbit planes, thereby reducing launch costs by 30% or more, and potentially saving hundreds of millions of dollars for a constellation of twenty-four spacecraft.
Unfortunately, and as shown below in Tables 1 and 5, this mission is not presently viable using conventional orbit transfer systems. Specifically, because of the low specific impulse (Isp) of the Liquid Apogee Engine (“LAE”) and high propulsion system dry mass, a negative mass margin of approximately 225 kg occurs for a spacecraft with a dry mass of 2249 kg. Known approaches to increase orbit transfer efficiency, such as partial orbit transfers with Hall Current Thrusters (“HCTs”), cannot be used, because the spacecraft would spend too much time in the proton belts, possibly damaging its solar array and electronics.
TABLE 1Mission Results for Conventional SystemsConventional PropulsionQuantitySystemSpacecraft dry mass (kg)2249Max. dry mass to orbit (kg)2024Spacecraft mass margin (kg)−225Mission viabilityNoFuel mass (kg)2083Oxidizer mass (kg)1982SC wet mass (kg)6099LAE specific impulse (sec)329LAE mixture ratio1LAE thrust (lbf)170Orbit transfer efficiency98%LEO storage orbitAlt 1,408 km circ., 39deg inc.MEO mission orbitAlt 20,182 km, 55 deginc.Launch vehicleAtlas V551Dual launch payload system13,150weight (“PSW”) (kg)
One approach is to use a conventional pump-fed rocket engine in order to get higher inlet pressures and a higher Isp. Since conventional pump-fed rocket engines use turbopumps that operate off of decomposed fuel, such as hydrazine gas, fuel efficiency is reduced since some of the fuel is consumed by the pumps. Furthermore, since the pump is an integral part of the LAE, new pump designs must be developed if the LAE is changed, increasing complexity and cost.
Accordingly, it is desirable to provide for spacecraft propulsion systems which overcome these conventional deficiencies. In particular, it is desirable to provide for an enhanced performance, high-efficiency spacecraft propulsion system which increases the spacecraft orbit transfer performance with modifications that increase LAE efficiency and reduce propulsion system mass.